1. Field of the Invention
The present invention concerns a natural commutation static frequency converter intended to supply a synchronous motor, and more particularly a converter of the type comprising a polyphase cycloconverter with control circuit, including n monophase cycloconverters each having two reversed Graetz bridges of which the inputs are interconnected as well as the outputs, one of the outputs of each cycloconverter being connected to a neutral point.
2. Discussion of the Prior Art
Natural commutation static frequency converters have the advantage, as their name indicates, of not using forced commutation circuits, which complicate and weigh down the converter, for extinction of thier controlled semiconductor elements (e.g. thyristors). Reversal of the voltage at the terminals of the thyristors induces extinction. For these extinction conditions to be present, it is necessary, for example, that the motor supplied by the converter have a forward cos .psi. (the current phase is advanced with regard to the voltage phase) and sufficient voltage.
There exist two principal types of static frequency converters, i.e. rectifier-inverters and cycloconverters.
A three-phase natural commutation rectifier-inverter supplying a synchronous motor is perfectly satisfactory when the speed of the motor is high enough, but natural commutation does not occur at low speeds due to the lack of voltage. It is then necessary momentarily to cut the inverter current with the rectifier when the combination of thyristors crossed by the current at the inverter must be changed. The result is a time-scalloped couple having a lower average value where the maximal couple is generally required.
By contrast, a cycloconverter functions properly at low speeds of the motor but requires an input frequency two or three times higher than the frequency of the motor. This constraint may become troublesome, indeed even fatal, if the frequency of the motor is high.